[readme] use single quotes in nested ternary bad example (section 15.6)

The bad example in section 15.6 (nested ternaries) used double-quoted
string literals ('"bar"', '"baz"'), which contradicts section 6.1 of
this same style guide that requires single quotes.

Also related to issue #3152 which flagged inconsistent style conventions
in code examples across sections.

Fixes the bad example to use single-quoted strings consistently with
the rest of the guide and with the good examples immediately below.